Terrace Length Research Articles

Vicinal (111) surfaces at Si solid-liquid interface during unidirectional solidification

The solid-liquid interfaces of silicon grains have been observed by an in-situ system. Two silicon seeds having different crystallographic orientations (as ⟨100⟩, ⟨110⟩, and ⟨111⟩) along the growth direction were placed side by side in a silica crucible for the direct comparison of interface behavior during the melt-growth process. Experimental evidence proved the existence of flat {111} surfaces at different grain orientations during crystallization. The difference in the interface positions of the two grains was used to calculate the undercooling of the {111} surfaces. The growth rate-undercooling relationship was linear in all experiments, showing that the observed flat surfaces were planes vicinal to {111} facets, growing through a step flow mode. The terrace length was constant but different from one experiment to another. Therefore, this work indicates that it is crucial to consider the vicinal surface kinetics in the analysis of Si facet planes and grooves during the melt-growth process.

Dynamic of centrifugal step emulsification and prediction of droplet diameter

Studies show that centrifugal step emulsification is an effective method for high throughput droplet generation and has been widely used over the past ten years. However, there is no in-depth understanding of the physics underlying emulsification and the effect of centrifugal force on the droplet volume. Aiming at resolving this shortcoming, this article is focused on the dynamics of the droplet formation process subjected to centrifugal acceleration, and a theoretical model is proposed for accurately predicting the droplet size. A critical time and a critical bulb length are introduced to describe the droplet formation and divide this process into stable and rapid filling regimes. It is worth noting that the centrifugal force was considered in the dispersed phase profile. Finally, a theoretical model was established to predict the droplet size. Numerical simulation and high-speed measurements demonstrate that there is a critical time and bulb length, and the critical bulb length is in good agreement with the proposed theory. To evaluate the performance of the model, experiments with different centrifugal accelerations, terrace heights, and terrace lengths were carried out. The obtained results are in excellent agreement with the experiments, and the relative diameter error was less than 4%. The performed analyses demonstrate that the established model can be applied to accurately calculate the droplet size and obtain the correlation between the droplet size and different parameters, such as terrace height, terrace length, and centrifugal acceleration. This model has great potential in guiding the designs of centrifugal step emulsification systems.

Coherent X-ray measurement of step-flow propagation during growth on polycrystalline thin film surfaces

The properties of artificially grown thin films are strongly affected by surface processes during growth. Coherent X-rays provide an approach to better understand such processes and fluctuations far from equilibrium. Here we report results for vacuum deposition of C60 on a graphene-coated surface investigated with X-ray Photon Correlation Spectroscopy in surface-sensitive conditions. Step-flow is observed through measurement of the step-edge velocity in the late stages of growth after crystalline mounds have formed. We show that the step-edge velocity is coupled to the terrace length, and that there is a variation in the velocity from larger step spacing at the center of crystalline mounds to closely-spaced, more slowly propagating steps at their edges. The results extend theories of surface growth, since the behavior is consistent with surface evolution driven by processes that include surface diffusion, the motion of step-edges, and attachment at step edges with significant step-edge barriers.

Investigation of the interfacial properties of platinum stepped surfaces using peroxodisulfate reduction as a local probe

Peroxodisulfate (PDS) reduction has been investigated by electrochemical and FTIR techniques in a wide pH range, on platinum stepped single crystals electrodes vicinal to the (111) pole. PDS reduction is a structure-sensitive reaction and proceeds on well differentiated potential regions for terrace and step sites. This fact allows gaining local information about the individual behavior of those sites. The reaction proceeds at appreciable rate in a narrow potential range, being inhibited in the low and high potential regions of the voltammogram. The inhibition of the reaction at low potentials is most likely associated with a change in the sign of the charge on the electrode, from positive to negative. Therefore, PDS reduction gives an approximate indication of the position of the potential of zero free charge (pzfc). In this way, two local values of pzfc, associated with terrace and steps sites, can be determined. Such values of local pzfc are compared with the local values of potential of maximum entropy (pme) of double layer formation deduced from laser induced T-jump experiment. Good agreement between both magnitudes is found, especially for terraces. The local pzfc on terraces shifts to more positive potentials as the terrace length diminishes, while the local pzfc for steps remains constant. The inhibition of PDS reduction at high potentials can also be related with a second inversion in the sign of the electrode charge. Therefore, PDS reduction can be used to determine a second pzfc on the (111) terraces at high potentials, revealing a non-monotonic charge – potential relationship. Spectroscopic experiments allow the identification of the species involved in PDS reduction.

A theory of growing crystalline nanorods – Mode I

Nanorods grow in two possible modes during physical vapor deposition (PVD). In mode I, monolayer surface steps dictate the diameter of nanorods. In mode II, multiple-layer surface steps dictate the diameter, which is the smallest possible under physical vapor deposition [5,10]. This paper reports closed-form theories of terrace lengths and nanorod diameter during the growth in mode I, as a function of deposition conditions. The accompanying lattice kinetic Monte Carlo simulations verify these theories. This study reveals that (1) quasi-steady growth exists for each set of nanorod growth conditions, and (2) the characteristic length scales, including terrace lengths and nanorod diameter at the quasi-steady state, depend on the deposition conditions – deposition rate F, substrate temperature T, and incidence angle θ – only as a function of l2D/tan θ, with l2D=2(v2DFcosθ)13 as a diffusion-limited length scale and v2D as the atomic diffusion jump rate over monolayer surface steps.

Effect of pH and Water Structure on the Oxygen Reduction Reaction on platinum electrodes

The oxygen reduction reaction (ORR) at different pH values has been studied at platinum single crystal electrodes using the hanging meniscus rotating disk electrode (HMRDE) configuration. The use of NaF/HClO4 mixtures allows investigating the reaction up to pH=6 in solutions with enough buffering capacity and in the absence of anion specific adsorption. The analysis of the currents shows that the kinetic current density measured at 0.85V for the Pt(111) electrode follows a volcano curve with the maximum located around pH=9. This maximum activity for pH=9 can be related to the effects of the electrode charge and/or water structure in the ORR. On the other hand, the catalytic activity for the other basal planes shows a monotonic behavior with a small dependence of the activity with pH. For stepped surfaces with (111) terraces, the behavior with pH changes gets closer to that of the Pt(111) surface as the terrace length increases. Additionally, the ORR curves show a dependence of the limiting diffusion current with pH. It is observed that the limiting current density diminishes as the pH increases in a potential region where hydrogen peroxide is readily reduced. These results suggest the existence of a bifurcation point in the mechanism previous to peroxide formation, in which OOH• is proposed as the bifurcation intermediate. The reduction of OOH• requires proton addition and would be more difficult at neutral pH values, justifying the diminution of the limiting currents.

Factors influencing the initial establishment of salt marsh vegetation on engineered sea wall terraces in south east England

Sea walls provide vital flood protection for lowland coastal property. We investigated the integrity of a cost-effective method of repairing sea defences, which has potential to create habitat for coastal and salt marsh flora. Experimental stone-gabion and clay-filled terraces were installed as a soft engineered approach to repair damaged sea walls in estuarine embayments in south east England. Changes in the surface heights of sediment and vascular plant colonisation were monitored over a 22 month period. Seven of the 12 terraces were colonised, by 12 species of plant, reaching a maximum of 85% cover. The main drivers of plant colonisation were sediment stability, elevation, exposure and sediment shear strength. Terraces with least change in the surface height of sediments were favourable for plant colonisation. Ordination (Canonical Correspondence Analysis) showed 72% variation in plant distribution explained by elevation (37%), exposure (30%), terrace length and sediment shear strength (5%). Elevation was the most influential variable; recruitment increased as terrace height approached the height of existing marsh (r2 = 0.43). This cost-effective approach has the potential to provide protection to sea walls and create additional habitat for wildlife. Key considerations for the improvement of terrace design and construction are discussed.

β-lactoglobulin as food grade surfactant for clove oil-in-water and limonene-in-water emulsion droplets produced by microchannel emulsification

Unheated and heat-aggregated β-lactoglobulin (β-lg) solutions were used to stabilize clove oil-in-water emulsion and limonene-in-water emulsion prepared by microchannel (MC) emulsification. The size of the MC array plate was 15 × 15 mm2 consisting of 100 parallel MCs fabricated on each side of the plate. The channels were 4 μm depth, 71 μm length, and 8.2 μm width, with the terrace length of 29.1 μm. Unheated and heat-aggregated β-lg effectively stabilized clove oil-in-water and limonene-in-water emulsions during production using MC emulsification. The emulsion droplets were steadily produced by the channels. The average diameter of clove oil droplets was around 17 μm, with both unheated and heat-aggregated β-lg, regardless of the concentration of β-lg. The average diameter of the limonene droplets was 18–26 μm depending on the conditions (unheated or heat-aggregated) and the concentration of β-lg. The coefficient of variation for all average droplet diameters was ≤8%, which was an indication of monodisperse droplets. Clove oil-in-water emulsion droplets became polydisperse within 2 h observation, whilst limonene-in-water emulsion droplets remained monodisperse over time. The stability of the oil droplets over time related to the solubility of the oil phases rather than the effects of β-lg as the surfactant. In addition, the droplet sizes at different surfactant concentrations seemed to relate to the viscosity ratio between the dispersed phase and the continuous phase rather than the effects of the surfactant.

Numerical model predictions of autogenic fluvial terraces and comparison to climate change expectations

AbstractTerraces eroded into sediment (alluvial) and bedrock (strath) preserve an important history of river activity. River terraces are thought to form when a river switches from a period of slow vertical incision and valley widening to fast vertical incision and terrace abandonment. Consequently, terraces are often interpreted to reflect changing external drivers including tectonics, sea level, and climate. In contrast, the intrinsic unsteadiness of lateral migration in rivers may generate terraces even under constant rates of vertical incision without external forcing. To explore this mechanism, we simulate landscape evolution by a vertically incising, meandering river and isolate the age and geometry of autogenic river terraces. Modeled autogenic terraces form for a wide range of lateral and vertical incision rates and are often paired and longitudinally extensive for intermediate ratios of vertical‐to‐lateral erosion rate. Autogenic terraces have a characteristic reoccurrence time that scales with the time for relief generation. There is a preservation bias against older terraces due to reworking of previously visited parts of the valley. Evolving, spatial differences in bank strength between bedrock and sediment reduce terrace formation frequency and length, favor pairing, and can explain sublinear terrace margins at valley boundaries. Age differences and geometries for modeled autogenic terraces are consistent, in cases, with natural terraces and overlap with metrics commonly attributed to terrace formation due to climate change. We suggest a new phase space of terrace properties that may allow differentiation of autogenic terraces from terraces formed by external drivers.

Thermodynamic properties of hydrogen–water adsorption at terraces and steps of Pt(111) vicinal surface electrodes

In this work, the effect of temperature on the adsorption states of Pt(111) vicinal surface electrodes in perchloric acid is studied through a thermodynamic analysis. The method allows calculating thermodynamic properties of the interface. In this framework, the concept of the generalized isotherm and the statistical thermodynamics description are applied to calculate formal entropies, enthalpies and Gibbs energies, ΔG¯i0, of the adsorption processes at two-dimensional terraces and one-dimensional steps. These values are compared with data from literature. Additionally, the effect of the step density on ΔG¯i0 and on the lateral interactions between adsorbed species, ωij, at terraces and steps is also determined. Calculated ΔG¯i0, entropies and enthalpies are almost temperature-independent, especially at steps, but they depend on the step orientation. In contrast, ΔG¯i0 and ωij at terraces depend on the step density, following a linear tendency for terrace lengths larger than 5 atoms. However, while ΔG¯i0 increases with the step density, ωij decreases. Results were explained by considering the modification in the energetic surface balance by hydrogen, Hads, and water, H2Oads, co-adsorption on the electrode, which in turn determines the whole adsorption processes on terraces and steps.

How Well Does Pt(211) Represent Pt[n(111) × (100)] Surfaces in Adsorption/Desorption?

We have investigated to what extent Pt(211) is representative for Pt[n(111) × (100)] surfaces in adsorption/desorption behavior of water, hydrogen, and oxygen through temperature-programmed desorption. In contrast to surfaces with n > 3, H2O adsorbs to Pt(211) in a crystalline fashion far below the usual crystallization temperature of amorphous solid water. For D2, we find that desorption from (100) steps is independent of terrace length for n ≥ 3, but desorption from the neighboring (111) terraces varies. Larger terraces result in larger variations in binding energies as a consequence of decreasing proximity of adsorption sites to the step edge. For O2, we observe enhanced dissociation on Pt(211) resulting in a much larger maximum O coverage than surfaces with n > 3. The TPD characteristics suggest formation of 1D PtO2 structures, which are only formed for n = 3 with this (100) step type. Hence, Pt(211) can by no means be considered representative of Pt(111) terraces truncated by (100) steps. Our results...

An Online Planning Tool forDesigning Terrace Layouts

<abstract><title><italic>Abstract. </italic></title> A web-based conservation planning tool, WebTERLOC (web-based TERrace LOCation program), was developed to provide multiple terrace layout options using digital elevation model (DEM) and geographic information systems (GIS). Development of a terrace system is complicated by the time-intensive manual layout process, which can take up to 50% of the total design time. The objective was to develop a web-based conservation planning tool for designing terrace layouts. The use of WebTERLOC enables automation of the terrace layout procedure, potentially saving time and money. Functionality of the program was enhanced by integration with an online ArcGIS interface. Multiple help menus and automated drawing and uploading tools increased input efficiency and ease of program use. Based on an earlier version of TERLOC, existing design procedures were modified to enhance layout results for fields with irregular boundaries, topography, and orientation. Program design modifications included variable terrace spacing, channel grade adjustments to meet non-erosive velocities, and the option for multiple key terraces. These new features provide means of selecting layout designs based on conservation, overall terrace length, and farmability. Analysis of WebTERLOC and its output was performed by comparing the program layout results for five fields with manually located terraces using standard NRCS procedures. Program results were comparable to the manual terrace layout results, with fields having large drainage areas most similar. Terrace number, shape, and length were the features with the most similarities. Terrace spacing was found to be most variable, with the program spacing typically larger than the manual result, particularly for smaller, more uniform fields. For example, fields less than 10 ha resulted in program spacings 25% to 67% greater than manual layouts, while larger fields were 4% to 22% greater. Depending on location of the uppermost terrace in larger fields, total field terrace numbers were equal or within one or two of the constructed layout. Excluding these uppermost or lowest terraces additional terraces, total field terrace lengths were within 1% to 3% of the constructed lengths. In general, the WebTERLOC program successfully demonstrated its use as an efficient terrace layout planning tool.

Scattering of surface electrons by isolated steps versus periodic step arrays

We investigate the scattering of electrons belonging to Shockley states of (111)-oriented noble metal surfaces using angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM). Both ARPES and STM indicate that monatomic steps on a noble metal surface may act either as strongly repulsive or highly transmissive barriers for surface electrons, depending on the coherence of the step lattice, and irrespectively of the average step spacing. By measuring curved crystal surfaces with terrace length ranging from 30 to 180 \AA{}, we show that vicinal surfaces of Au and Ag with periodic step arrays exhibit a remarkable wave function coherence beyond 100 \AA{} step spacings, well beyond the Fermi wavelength limit and independently of the projection of the bulk band gap on the vicinal plane. In contrast, the analysis of transmission resonances investigated by STM shows that a pair of isolated parallel steps defining a 58 \AA{} wide terrace confines and decouples the surface state of the small terrace from that of the (111) surface. We conclude that the formation of laterally confined quantum well states in vicinal surfaces as opposed to propagating superlattice states depends on the loss of coherence driven by imperfection in the superlattice order.

Long-term agrarian landscapes in the Troodos foothills, Cyprus

We investigate the temporal and environmental relationships between the terraced hill slopes of Politiko-Koloiokremmos and the adjacent Bronze Age settlement of Politiko-Troullia in foothills of the Troodos Mountains, central Cyprus. Mapping of 102 stone walls on Koloiokremmos is compared with 66 walls farther afield on Cyprus to create a six-part terrace typology. Sherd counts from 174 2-m radius collection circles over approximately 20 ha on Troullia and Koloiokremmos were used to create interpolated sherd density surfaces revealing distinctly different patterns of land use for four major archaeological eras between the Cypriot Bronze Age and Medieval Period. We compared sherd density patterns according to terrace types, length, slope and vegetation cover determined by SAVI (Soil-Adjusted Vegetation Index) from Ikonos and Quickbird satellite imagery.The most robust sherd patterning indicates Prehistoric Bronze Age settlement at Troullia, as confirmed by soil resistivity and excavation, and multi-period land use on Koloiokremmos immediately upslope of the village that may be associated with agricultural terracing. Scattered sherd concentrations indicate later use of this landscape as an agricultural hinterland for the Iron Age city of Tamassos. Sherds and a down slope spread of roof tiles from the Roman through Medieval periods suggest an isolated structure near the crest of Koloiokremmos. Longer terraces correlate with greater vegetation cover and are interpreted as agricultural, while shorter terraces, apparently for erosion control and arboriculture, are significantly related to Prehistoric Bronze Age sherd patterns. Our results suggest that coordinated analysis of archaeological, geographical and remotely sensed environmental data associated with terraced landscapes can be used to infer long-term patterns of agricultural land use.

Kinetics of dissociative adsorption of ethylene glycol on Pt(s) − [ n(100) × (111)] electrodes in acid solutions

The dissociative adsorption of ethylene glycol (EG) on stepped surfaces of Pt single crystal (Pt(s) − [ n(100) × (111)]) was investigated. Different surface structures of these platinum single crystal electrodes were obtained by various treatment conditions. The results illustrated that the electrocatalytic activity of Pt single crystal electrodes towards EG dissociative adsorption is increased with the increase of (100) terrace length on the surfaces. It has been found that the different pretreatment hardly influences the reactivity of stepped Pt(s) − [ n(100) × (111)] surfaces towards EG dissociative adsorption. The results indicated that all studied surfaces are more active in perchloric acid than in sulfuric acid, and this effect is decreased with the increase of (100) terrace length on the surfaces, demonstrating the strong adsorption of (bi)sulfate anions on (111) sites and their weak adsorption on (100) sites. This study has gained knowledge on the interaction between EG and Pt single crystal electrodes, and thrown a new insight into understanding the fundamental of electrocatalysis and surface processes of EG dissociative adsorption.

Morphology of epitaxial SrTiO3/Si (001) determined using three-dimensional diffraction profile analysis

Large scale features of epitaxial films, such as terrace structure, strain distribution, and grain shape, can have a substantial effect on device properties. The diffraction spot shape captures the average large scale structure. In epitaxial SrTiO3/Si (001), the spot shows a L-dependent splitting along two axes. Analysis of this feature traces its origin to a jagged step edge formation, and determines the length scale of terrace length and step meandering. Such a structure was previously observed in dimerized Si (001) using real-space imaging. Si diffusion that occurs during the first stages of growth gives rise to this morphology. This morphology may play a key role in explaining the small critical thickness for relaxation in these films.

Plasmon damping below the Landau regime: the role of defects in epitaxial graphene

The sheet plasmon in epitaxially grown graphene layers on SiC(0001) and the influence of surface roughness have been investigated in detail by means of low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). We show that the existence of steps or grain boundaries in this epitaxial system is a source of strong damping, while the dispersion is rather insensitive to defects. To the first order, the lifetime of the plasmons was found to be proportional to the average terrace length and to the plasmon wavelength. A possible reason for this surprisingly efficient plasmon damping may be the close coincidence of phase (and group) velocities of the plasmons (almost linear dispersion) with the Fermi velocity of the electrons. Therefore, uncorrelated defects like steps only have to act as a momentum source to effectively couple plasmons to the electron–hole continuum.

A Geometric Model for the Dynamics of Microchannel Emulsification

Microchannel emulsification is an interfacial tension driven method to produce monodisperse microdroplets, or microspheres. In this paper we introduce a model for describing the dynamics of microchannel emulsification based on simple time dependent geometric shape analysis. The model is based on mechanistic principles that simultaneously predicts both process and microchannel geometry effects. The model contains no adjustable (fit) parameters and is thus fully predictive for oil in water emulsification. The model is easy to use and does not require extensive computational time and/or memory. The model was validated by comparison with the experimental results published by Sugiura and co-workers and we found excellent agreement. It was found that the droplet size of oil in water emulsions could be fully predicted using only two dimensionless numbers, an adapted capillary number that also comprises effects of terrace width and height, and the ratio of terrace length over terrace height. Based on these findings, a dimensionless design map could be constructed for a wide range of process conditions and microchannel dimensions.

Ethanol electro-oxidation on platinum in alkaline media

The electrochemical oxidation of ethanol as well as its irreversible adsorption on platinum single crystal electrodes has been studied in an alkaline solution. In addition, the adsorbed species in the ethanol oxidation reaction were also studied by means of surface enhanced Raman spectroscopy (SERS) on a platinum film electrode. It was found that the oxidation of ethanol is very sensitive to the electrode surface structure: a higher concentration of low-coordination sites increases the current, lowers the overpotential required and lowers the deactivation rate. Furthermore, the terrace length also affects the amounts and the nature of the adsorbed species: on Pt (110), only CO(ad) was observed, whereas adsorbed CH(x) could only be found on electrode surfaces with (111) terrace sites. Based on the results here, a model for the ethanol electro-oxidation reaction in alkaline media is presented, and the differences with the same reaction in acidic media are pointed out.

Step-flow growth of homoepitaxial ZnO thin films by ultrasonic spray-assisted MOVPE

Homoepitaxial zinc oxide thin films with atomically flat surfaces have successfully been grown on Zn-polar (0 0 0 1) ZnO substrates with the step-flow growth mode by ultrasonic spray-assisted metalorganic vapor phase epitaxy. The surfaces of the grown layers were composed of terraces and steps, where the step height was mainly 0.26 nm originated from half of a c-axis lattice parameter of hexagonal ZnO crystal, at the optimizing growth conditions. The step-flow growth required the higher growth temperature (for example, at 950 °C), while at lower temperatures (for example, at 875 °C) the growth mode was a mixture of layer-by-layer and step-flow growths. In order for the step-flow growth maintaining the monolayer steps, the miscut angles of substrates should be so small that the terrace lengths were about 200 nm or more in our experimental conditions.